1. Field of the Invention
The present invention relates to a power steering apparatus which applies a steering assist force to a steering mechanism by a hydraulic pressure generated by a pump driven by an electric motor.
2. Description of Related Art
Power steering apparatuses are conventionally utilized which assist operation of a steering wheel by supplying a working oil from an oil pump to a power cylinder coupled to a steering mechanism. The oil pump is driven by an electric motor, and a steering assist force is generated by the power cylinder in accordance with the rotation speed of the electric motor. The steering assist force is not required when the steering wheel is not turned. Therefore, the electric motor is off in a straight travel steering state (steering inactive state) where the steering wheel is virtually at a steering angle midpoint and, in response to detection of a change in any of steering-related conditions, the electric motor is actuated.
For power saving and smooth rise of a hydraulic pressure, a standby driving operation is performed to drive the electric motor at a low voltage in the steering inactive state.
In the electric motor control described above, it is necessary to detect a time point at which a transition from a steering active state to the steering inactive state occurs.
Japanese Unexamined Patent Publication No. 5-69844 (1993), for example, discloses a motor control method in view of the fact that an electric current flowing through the electric motor varies in accordance with a steering torque. In this method, the motor current is sensed and, when the absolute value of the sensed electric current or the change amount of the sensed electric current relative to a non-load electric current is reduced to lower than a threshold, the electric motor is stopped or driven at the minimum rotation speed.
However, a curve plotted between the motor current and the steering angular position is generally shifted in dependence on a vehicle speed as shown in FIG. 6. Therefore, a steering angular position at which a transition to a motor off state or a standby state occurs varies depending on the vehicle speed, if the judgment for the detection of the transition timing is based on a predetermined electric current threshold .DELTA.I. This results in variations in steering operation feeling.